At the mean time the site has been taken down and administrator are investigating the hack. According to Bitcointalk admin Theymos, the JavaScipt injected into the site appears to be harmless, but it’s possible that the hackers gained access to the database.
“There's a good chance that the attacker(s) could have executed arbitrary PHP code and therefore could have accessed the database, but I'm not sure yet how difficult this would be,” Theymos noted on Reddit.
As far as concerned about the customer security, Theymios have advised to users to change the password of the accounts. As forum was quite popular, hence many on its users may had got affected.
Regarding the hack, Bitcoinstalk admin have announced to reward 50 Bitcoins to the first individual who can pinpoint the security hole.
If you want to see the how was the hack. check the below video.